It is known to remove sodium sulfate from a sulfate-containing sodium chloride solution by various techniques. Illustrative of this type of prior art are U.S. Pat. Nos. 3,712,797 to Winkler and 2,125,624 to Davis et al. Davis et al chill natural brine to about 30.degree. F. to precipitate sodium sulfate in a relatively pure state. Included in these type of techniques are direct evaporation of brine to concentrate the brine followed by sodium sulfate removal by precipitation. This latter type of process requires either large storage areas to pool the brine for solar evaporation, as well as a long amount of time, or requires a tremendous expenditure of energy to produce the heat necessary for evaporative concentration.
The relative solubility of sodium sulfate and sodium chloride has been investigated to some extent in the prior art. The Davis et al and Winkler patents, discussed above, show this type of art. Winkler reports at column 1, lines 59-63, that the solubility behaviours of sodium chloride and sodium sulfate vary inversely with fluctuating temperature, that is, with increasing temperature, sodium chloride solubility rises and sodium sulfate solubility falls; whereas with decreasing temperature, sodium chloride solubility falls and sodium sulfate solubility rises.
The use of a precipitating agent such as methanol to recover sodium sulfate from a sulfuric acid solution containing sodium sulfate is known. U.S. Pat. No. 873,070 to Nibelius is illustrative thereof. It is also known to recover anhydrous sodium sulfate by the addition of a precipitating agent such as methanol to a saturated solution of sodium sulfate. U.S. Pat. No. 2,482,830 to Allen, Jr., is exemplary of this type of prior art process. Furthermore, it is known to recover anhydrous sodium sulfate by the addition of methanol to a sulfuric acid solution of sodium sulfate. Illustrative of this type of prior art is U.S. Pat. No. 3,331,661 to Boiston et al.
Other prior art showing the use of aqueous methanol to cause crystallization of sodium sulfate is exemplified by U.S. Pat. No. 3,597,168 to Hoppe et al. This patent uses the methanol to produce large grain crystals of sodium sulfate from a saturated aqueous solution of sodium sulfate.
It is also known in the prior art to add methanol to sea water brine having a specific gravity of about 1.31 so as to separate potassium sulfate and magnesium sulfate as a double salt. U.S. Pat. No. 3,925,028 to Lozano is representative. In this process, some sodium sulfate precipitates along with the potassium and magnesium sulfate. Lozano provides an about 72 volume percent concentration of methanol in the brine-methanol mixture.
This prior art and the other prior art of which we are aware is deficient because it fails to provide an energy efficient process for removing sulfate ion from an aqueous sodium chloride solution, in a process for separating zirconium and hafnium; because it fails to provide a process for recovering sodium sulfate having a purity greater than 99.9% from a sulfate ion-containing sodium chloride solution produced as a waste product in a process for separating zirconium and hafnium; and because it fails to provide a combination process that is energy efficient with regard to removing sulfate ion and that provides at the same time sodium sulfate of greater than 99.9% purity, in a process for separating zirconium and hafnium.